1. Field of the Invention
The present invention relates to an image processing apparatus. Particularly, the present invention is suitable to correct, for each observer, aberrations generated in an image display apparatus (e.g., HMD or EVF) having a display optical system which enlarges an image formed on a display device and allows an observer to observe the enlarged image.
2. Description of the Related Art
Many techniques have been proposed to digitally correct distortion of an image and color misregistration (bleeding) caused by aberrations generated by an optical lens. A lens for canceling aberrations is newly necessary to optically correct the distortion and color misregistration, increasing the size of the optical system. A lens with good optical characteristics designed to prevent these aberrations is generally expensive. Instead of such a lens, aberrations generated by an optical system are generally corrected by a digital aberration correction technique using image processing. This technique can downsize the optical system (decrease the number of lenses) and employ an inexpensive lens.
Recently, image capturing apparatuses such as digital cameras having more compact, lighter-weight, lower-cost main bodies require reductions in the size and cost of imaging lenses. Further, image capturing apparatuses such as digital cameras generally have zoom and focusing functions. Demand has arisen for a correction technique of maintaining a high image capturing quality regardless of a change of aberrations upon a change of lens parameters. Japanese Patent Laid-Open No. 2006-135805 (to be referred to as patent reference 1 hereafter) discloses an aberration correction technique capable of removing an image blur caused by chromatic aberration of magnification based on lens parameters even when an image is captured while the lens parameters including the zoom, focus, and iris are changed to various values. The technique disclosed in patent reference 1 can suppress degradation of the image capturing quality, i.e., an image blur by correcting chromatic aberration generated by the imaging lens upon a change of lens parameters in an image capturing apparatus such as a digital camera.
Many recent digital cameras include an electronic view finder (to be referred to as an EVF). The EVF is known as a technique of making an image confirmed via the viewfinder before image capturing truly match a captured image. The EVF adopts an arrangement using a digital image sensor to eliminate a reflex mirror (known as a reflector) indispensable for a single-lens reflex camera. The EVF is configured to display an image sensed by a digital image sensor in real time on a compact display device, enlarge the image formed on the display device via a display lens, and allow an observer to observe it.
The same aberrations as the above-mentioned ones of an imaging lens occur even in a display optical system which enlarges an image formed on a compact display device via a lens and allows an observer to observe it, like the EVF. This display optical system also requires an aberration correction technique of correcting degradation of the display image quality caused by distortion and color misregistration (bleeding).
A head mounted display (to be referred to as an HMD) is another example of the image display apparatus which enlarges an image formed on a compact display device and allows an observer to observe it. Many proposals have been made for a technique of applying a digital aberration correction technique to an HMD or EVF to prevent degradation of the display image quality caused by aberrations generated by the optical system of a display apparatus.
However, these conventional techniques suffer the following problems.
In addition to the display optical system of an HMD or EVF, another optical system may exist on an optical path through which light of an image formed on a display device reaches the pupil of an observer when he observes an image via the HMD or EVF.
For example, in aberration correction to correct only aberrations generated by the display optical system of the HMD or EVF, an aberration (color misregistration) is appropriately corrected for an observer who has good vision and can observe with his naked eyes. The observer can observe a display image maintaining the quality. However, when an observer who uses a powerful vision correction optical system observes the same corrected image, he observes the color misregistration even on the corrected image. This means that an appropriate aberration correction amount changes depending on whether the observer uses a vision correction optical system and the difference in the degree of vision correction.
That is, in observation via an HMD or EVF, generated aberrations need to be corrected in accordance with a combination of the display optical system of the HMD or EVF and a vision correction optical system used by an observer. However, the conventional techniques cannot meet this requirement.
The degree of correction by a vision correction optical system used by an observer changes for each observer. An appropriate aberration correction amount needs to be selected or generated for each observer when correcting aberrations generated by a combination of the display optical system and the vision correction optical system, as described above.
Optical systems used by HMD and EVF observers include vision correction optical systems (spectacle lenses and contact lenses). The degree of vision correction varies depending on the vision of each observer. There are known many methods for measuring the optical characteristics (e.g., refractivity, aberrations, and spatial distribution of them) of a vision correction optical system (mainly a spectacle lens), including a medical lens-meter used in for example, an eye clinic or by an optician. For example, Japanese Patent Laid-Open No. 06-018363 (to be referred to as patent reference 2) discloses a technique capable of measuring the refractivities of both spectacle lenses and contact lenses using only a single lens-meter. This technique adopts a moving means for moving a test lens along the optical axis when measuring a spectacle lens and measuring a contact lens. Japanese Patent Laid-Open No. 2002-228545 (to be referred to as patent reference 3) discloses a technique which uses a digital camera to capture the pattern image of an optical characteristic pattern observed via a lens and measure it as the optical characteristics of a frame-attached spectacle lens.
However, it is difficult to directly apply the above-mentioned conventional techniques to compact apparatuses such as an EVF and HMD to obtain optical characteristics including aberrations of a vision correction optical system. Hence, it is required to present a display image optimum for each observer by appropriately correcting aberrations generated by a combination of an optical system which allows an observer to observe an image formed on a display device, and a vision correction optical system used by the observer.